1. Field of the Invention PA1 The present invention relates to the manufacture of a magnetic recording medium which is typically used in the form of a magnetic recording tape, a floppy disc or the like, and more particularly to a method of smoothing magnetic films. PA1 2. Description of the Prior Art PA1 It is known that a web may be coated with a coating material while it is being moved, thereby forming a film over the surface of the web. In general, the thus-formed film is subjected to a smoothing process in order to make the surface of the film smooth and uniform. In the field of manufacture of magnetic recording media such as magnetic tapes and floppy discs, various processes for smoothing magnetic films are practiced in order to improve the performance of such media. PA1 A method in which a coated film is smoothed by contacting a doctor bar or a doctor knife to the surface of the film and in direct contact therewith has heretofore been widely used as a typical smoothing process, as disclosed in Japanese Patent Examined Publication No. 11336/1973. However, such a prior-art process involves disadvantage in that scratches or other defects are easily formed due to air movement or foreign matter which might have an influence during the contacting movement between such doctor element and the film and in that, since a web vibrates with respect to the doctor bar or doctor knife which is disposed in a fixed manner, the film formed on the web is likely to vary in thickness.
A film-smoothing method employing an air doctor is known as a method which does not involve such disadvantages to any substantial extent. This method has heretofore been utilized in smoothing a film formed by a coating paint having relatively low viscosity. Such a method is carried out in the following manner. A magnetic paint is coated on a web and a film is thus formed thereon. Before the film dries and solidifies, the wet film is supported and moved on a supporting roll and at the same time air is blown onto the film by means of an air doctor. The film is smoothed by the pressure of the air.
In another conventional method, while the film is being moved, a flexible film sheet is brought into contact with the surface of the film so as to smooth the latter, as disclosed in Japanese Patent Unexamined Publication No. 53631/1974. This method enables various improvements in erasure of cyclic unevenness in thickness, twill-like patterns and longitudinal stripes in reverse roll coating, as well as erasure of plate cylinder patterns in gravure roll coating. This brings about a significant improvement in the coated state of the film. Typically, in such a method, a magnetic paint is coated on a web while the latter is being moved, and a film is formed over the web. Before the film dries and solidifies, the surface of a flexible sheet is brought into contact with the wet film, the flexible sheet being secured at its one end to a fixed bar. Thus, the film is made smooth and uniform by the shear stress produced between the surface and the web.
In such a prior-art method, the air doctor has a significant effect in smoothing films formed of low-viscosity magnetic paint. It is, however, inferior with respect to magnetic paint having relatively high viscosity; for example, mottled patterns may be formed on the film.
Also, the aforesaid smoothing method employing a flexible sheet has advantage in that, since the sheet the ability to move in correspondence with the vibration which might occur during movement of the web, it is possible to reduce variations in film thickness. In general, this smoothing process is effective with respect to low-viscosity magnetic paint. However, since a magnetic paint containing fine grain magnetic particulates is commonly used for a magnetic recording medium of the type suitable for high-density recording, the viscosity of the paint per se is increased. It is therefore difficult for any of the prior-art methods to meet the demand for high-precision smoothness of such a film.
Such difficulty is typically derived from the fact that a magnetic paint of the aforementioned type exhibits large thixotropic properties, that is, exhibits large variations in viscosity in response to shearing; for example, the viscosity is reduced by imparting shear stress to the paint and also, if the shear stress is eliminated, the paint recovers its viscosity relatively quickly. Therefore, the paint coated on a web may be reduced in viscosity at the time that shear stress is imparted to the web during a coating process. However, the paint recovers its viscosity by virtue of its thixotropic properties before the smoothing step is effected following the coating step. In addition, since, after the coating, a diluting solvent evaporates from the film extremely quickly, another layer having relatively high viscosity is formed on the surface of the film. Accordingly, in this state, when air is blown onto the film through the nozzle of the air doctor, the surface layer having relatively high viscosity is partially broken, thereby producing small clumps. This constitutes a cause of formation of mottled patterns on the film. In order to prevent this phenomenon, consideration has been given to increasing the level of the air pressure of the air doctor. However, this measure produces no significant effect upon the occurrence of this phenomenon; rather, the increased air pressure may cause scattering of the paint which forms the film. The scattered paint may stick to the film, and this could result in lowering of the quality of smoothness.
Also, in a state wherein the aforesaid layer with relatively high viscosity is formed, if the flexible sheet is brought into contact with the web, the layer is subjected to shear stress between the web and the flexible sheet and thus its viscosity starts to decrease. However, before the viscosity of the film is reduced to a relatively low level uniformly in the direction of its thickness, the film passes by an associated edge of the flexible sheet. It is therefore impossible to obtain a uniform smoothing effect, and this may also result in the occurrence of longitudinal stripes, uneven thickness or mottled patterns.